ok here is a quick guess before i go look up the server settings.
Have you got the outgoing server set to "needs authentification" use same as incoming.?
That will screw you up because if there are messages to send on that account it tries to send them before it tries to read them and if its not set to authenticate you wont be logged in to send.
ATT is pretty weird about their email. There are any one of like 5 servers that should all work but usually only 1 of them does. And which one it is can vary from day to day it seems and they may have any particular account set up to use any particular server. The fact that they have yahoo running half of their email address's doesnt help either.
Strange as it sounds they may have given one of your accounts to yahoo and kept one on their own server even though they have the same domain name.
try using these settings
Email address:[email protected]
Incoming mail server (POP3): pop.att.yahoo.com
Incoming mail server SSL: enabled
Incoming mail port: 995
Outgoing mail server (SMTP): smtp.att.yahoo.com
Outgoing mail server SSL: enabled
Outgoing mail port: 465
Username: your username
Password: your password
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